Role of exogenous Ca in cellular signal transduction and Ca
homeostasis
Apart from acts as a nutrient element and structural component, Ca also
plays an intracellular second messenger role in plant, and participates
in a diverse of signal transduction pathways and biological
processes.9 Our study
showed that exogenous Ca markedly increased the expression of an
important Ca receptor and Ca signal transducer, calmodulin (CAM), at
both gene transcription level and proteomic level (Figure 5 and Table
1). Besides, the gene expression of other three Ca receptors and signal
transduction related proteins such as Ca-dependent protein kinase 1
(CDPK1 ), calcineurin B-like protein (CBL ) and touch 3 gene
(TCH3 ) were up regulated dramatically by exogenous Ca (Figure 5).
The roles of these Ca receptors in Ca mediated various signal
transductions, cellular events and biological processes including plant
growth and development have been widely
studied.9,11,69Adequate exogenous Ca supply ensured sufficient intracellular Ca
reservation and Ca ion homeostasis, which is the precondition of various
Ca involved signal transduction. On the contrary, Ca deficiency lead to
significant down-regulation of these genes (Figure 5), suggesting
various Ca signal transduction pathways may be distributed by Ca
deficiency, and thereafter result in intracellular Ca ion decline and Ca
ion dyshomeostasis.70Consistently, physiological parameter measurement (Figure 1D) showed low
Ca treatment decreased P. massoniana leaf Ca content while high
Ca treatment markedly increased leaf Ca content.
Calreticulin 3 (CRT3) and calnexin 1 (CNX1) are another two Ca binding
proteins, which localized to endoplasmic reticulum (ER) and involved in
the transportation and storage of Ca ion within
ER.71 Furthermore, the
two proteins also function as molecular chaperones in the regulation of
protein folding.72Exogenous Ca motivated higher expression of this two genes (Figure 5)
implies Ca homeostasis regulation in ER is active under adequate
exogenous Ca condition. Moreover, exogenous Ca may accelerate the
protein folding and packaging through activating these two molecular
chaperones, collaborating with other molecular chaperones such as HSP 70
and PPIase CYP20-2 (Table 1).